Vulcanization of rubber



Patented Oct. 27, 1936 UNITED STATES VULCANIZATION OF RUBBER Herman R.Thies, Akron, Ohio, assignor to Wingfoot Corporation, Wilmington, DeL, acorporation of Delaware N0 Drawing. Application March 18, 1932, SerialNo. 599,845

9 Claims.

This invention relates to improvements in the vulcanization of rubber.More particularly, it relates to the vulcanization of rubber underconditions such that scorching is reduced or eliminated and desirableproperties imparted to the product. It extends both to a new method ofvulcanizing and to a new rubber composition.

In the compounding of rubber, it is common practice to break down therubber and work the desired compounding ingredients into the same onarubber mill, a treatment which results in the evolution of aconsiderable amount of heat. With the recent introduction of internalmixing machines, still higher temperatures accompany the compoundingoperations. In the further processing, particularly the calendering andextruding operations, rather high temperatures are necessary.

In the case of rubber mixes containing the more active organicaccelerators, particularly ultraaccelerators, it frequently happens thattrouble is caused and loss occasioned by scorching; i. e. prematurevulcanization resulting from the high temperatures encountered duringand after compounding. In some cases, precautions are taken to reducethe danger of scorching by cooling the rubber during the compoundingstep or immediately thereafter.

The present invention provides an improved process of vulcanizing inwhich the danger of scorching, even with ultra-accelerators, is eitherminimized or eliminated entirely.

Briefly stated, it has been discovered that by the addition of a smallpercentage of a suitable aliphatic carboxylic acid or certain metallicsalts thereof to a rubber compound containing a semiultra orultra-accelerator, the tendency to scorch below temperatures of about260 degrees F. is greatly reduced or eliminated without, however,appreciably prolonging the time required for satisfactory vulcanizationat higher temperatures. Aliphatic carboxylic acids which are particular-1y valuable are crotonic, heptylic, tiglinic, linoleic, palmitic,lauric, oleic, dichlor acetic formic, caproic, capryllic, pelargonic,capric, fumaric, maleic, malic and adipic. Certain metallic salts ofthese acids are also quite valuable, particularly the zinc salt,specific examples being zinc oleate, zinc lactate, zinc oxalate, zinctartrate, zinc salt of fumaric acid and the zinc salt of malic acid. Itis of course to be understood that my invention is not limited to thesetypical members of the class, but includes numerous others also.

The amount of aliphatic carboxylic acid or metal salt thereof whichgives rise to the desired elimination or reduction of scorching issmall. In some cases. an amount as small as 6 of one percent, based onthe rubber, is sufficient; in others, amounts varying up to about threeper- 6iL cent, based on the rubber, give excellent results,

The amount will be found to vary with the particular aliphaticcarboxylic acid used, to some extent with the nature of the mix, andparticularly with the nature of the accelerator or acceleratorsemployed.

The more active organic accelerators which tend to cause scorchinginclude both the ultra accelerators and certain active accelerators notcommonly referred to in the industry as ultra-accelerators.Mercaptobenzothiazole, zinc dimethyl dithiocarbamate, the reactionproduct of mercaptobenzothiazole and diphenylguanidine, and thebutyraldehyde aniline condensation product prepared according to theprocess outlined in United States Patent No. 1,780,334 are illustrativeof these more active accelerators. These and other accelerators may beused alone or in various admixtures.

In the practice of the invention, the ingredients of the rubber mix maybe compounded in the customary manner, either externally on rubber rollsor internally in the Banbury mixer with the incorporation, in eithercase, of the aliphatic carboxylic acid in the mix so that it is presentduring the mixing operation and preferably before the addition of thesulphur. The rubber mix containing the aliphatic carboxylic acid isstable to a high degree: even though heated to some extent in the mixingapparatus, it nevertheless does not tend to scorch. It can be storedafter mixing without the necessity of cooling and without danger ofscorching.

The invention will be illustrated by the following examples, indicativeof the nature and advantages of the invention. In the tables, thefigures under the column index number are the times in minutes(determined by a solubility test) during which the rubber compounds wereheated to give a definite degree of scorch, as hereinafter explained.This test is a simple way of determining the scorch retarding powers ofthe various materials of the invention.

The index number may be determined by forming pellets from compoundedunvulcanized stock, the pellets being about /2 inch in diameter and ofuniform thickness. They are then exposed for various lengths of time toa constant temperature (in the tests 995 degrees C., a figurerepresenting the average mixing temperature), after which they areremoved from the heat and introduced into some inert rubber solvent (forexample, a high test gasoline) and allowed to remain for 15 minutes.Thereafter, they are violently agitated for a period of one minute.

If the stock is unscorched, a turbid suspension of rubber compound ingasoline is obtained; if scorched, this turbidity rapidly decreases. Thedecrease in turbidity is an indication of the degree of scorch and maybe measured by ascertaining the height of an obscuring column in anobscurometer, an instrument wherein the observer determines the heightof liquid column necessary to obscure a light filament of constantintensity.

One of the most common tests used by rubber technologists for many yearsfor determining scorch is the so-called hand feel test, in which, bypulling samples of the stock in question and bers were determined asabove outlined and the customary rubber tensile and elongation testsobtained. In all cases, a control or blank containing no aliphaticcarboxylic acid or zinc salt thereof was tested along with the examplescontaining one part by weight of the acid with the exception of dichloracetic acid of which only 0.25 part was added.

Index number at 995 0. Material Ultimate Tensile Maximum Elongationspecimen control specimen control Specimen Control Formic acid 126 166775 785 102 36 Dichlor acetic acid 237 189 760 725 101 51 Heptylic acid,210 206 770 750 61 36 Crotonic acid 166 164 760 725 73 46 Tiglinic acid200 164 785 726 65 46 Linoleic acid. 186 164 785 725 57 46 Palmiticacid. 192 166 730 785 48 36 Laurie acid 204 166 745 765 49 30 Oleic acid180 745 765 46 33 Zinc oleate- 207 200 745 765 55 28 Caproic acid. 224206 775 765 46 28 Caprillic acid 212 206 765 765 45 28 Oapric acid 230206 770 785 47 28 Pelargonic aci 236 206 785 765 39 28 Fumaric acid. 162166 785 765 143 36 Maleic acld 86 206 690 765 110 36 Malic acid 162 206765 765 66 36 Zinc salt of iumaric acid 196 805 775 45 36 Adipic ac' 192206 755 765 52 33 Zinc lactate 208 206 780 765 46 28 Zinc salt of malica 206 780 765 53 36 Zinc tartrate 184 206 765 765 46 36 Zinc oxalate 205206 750 765 40 28 observing its snap or elasticity, its. degree ofscorch is estimated. It has been found by a comparison of these gasolinesolubility tests and hand-feel tests that when the height of obscuringcolumn measures 100 mm., the scorching as measured by the hand-feeltests is barely perceptible. The index number is the time in minutes ofheating at 99.5 degrees C. at which the stock, when subjected to thegasoline solubility tests, gave a height of obscuring column of 100 mm.

It will also be seen from the tensile and elongation figures that theuse of these aliphatic carboxylic acids does not interfere materiallywith the curing properties of the stock. In certain cases, the valuesare even increased. It is of course necessary for best results, as hasbeen pointed out above, to use varying small amounts of acid fordiiferent rubber mixes, particularly for different accelerators. Theoptimum amount is best arrived at by experiment.

The following examples give the results obtained with the use ofmercaptobenzothiazole, an accelerator which is not commonly consideredto be an ultra-accelerator, but which, unless special precautions aretaken, has a tendency to cause scorching. Although nearly any of thestandard rubber formulae may be employed, the following, selected forthe sake of simplicity, has been found to be suitable.

Parts by weight Rubber (smoked sheet) 50 Rubber (pale crepe) 50 Zincoxide 10 Pigment 10 Sulfur 3 Mercaptobenzothiazole 1 In this compound,the pigment employed was rubber yellow #4, a yellow pigment produced bythe Federal Color Laboratory. Its use was simply to facilitate thedetermination of the index number.

To portions of this compound were then added the aliphatic carboxylicacids or zinc salts thereof in varying amounts, after which index num-It can be readily seen by comparison of the respective index numbers forthe compound and its control that the tendency of the stocks to scorchis greatly retarded by the presence of the above compounds.

Another compound in which these materials have been tested is thatoutlined in the following formula, which exemplifies a type of heavilyloaded compound advantageously used in rubber flooring stocks:

Rubber parts by weight 100.00 Loading volumes 124.00 Zinc oxide parts byweight 5.60 Sulfur do 4.00 Benzothiazyl disulfide do 1.50 Tetra methylthiuram disulfide do 0.15 Softener do 5.00 Antioxidant do 2.00

The 124 volumes of loading material included an inert filler, such asasbestine, and matter for coloring the rubber flooring material asdesired. The following data were obtained, each of the four stockshaving an optimum cure of 20 minutes at 285 degrees F.:

- Heated Amount Minutes Scorched Mateml added 99.5" o. fggl 130 0.

Control 31 10 6 Fumaric acid 3.0 parts 390+ 80 35 Maleic acid. 3.0 23449 18 Malic acid 3.0 61 18 9 Certain of the more heavily loaded stocksare known not to respond in respect of hardness to the ordinarycompounding procedures. In a white tile stock using 1.5 parts by weightmercapto benzo thiazyl disulfide and .15 part of tetra methyl thiuramdisulfide as an accelerating mixture, together with 4 parts sulfur and125 volumes of loading material, a durometer hardness of only 88 isobtained when cured at the optimum cure of 20 minutes at 285 degrees F.Varying the amounts of accelerators and vulcanizing agents to areasonable extent does not materially alter this value. The same effectis observable with respect to the resistance to bending, reasonableincreases of the curing ingredients offering no stiffer stocks than doesthe stock above cited.

I have found, however, that by adding 3 parts of an aliphatic carboxylicacid of the kind herein disclosed, the desired properties can beimparted to these stocks and the latter, at the same time, freed of thetendency to scorch.

The examples listed in the following table are typical of the class. Theincrease in hardness was measured by the Shore durometer, while thebending strength was measured by an Olsen machine, a well-known tensileand compression testing machine. The bending strength determinationswere made bytaking a small square of cured tile stock of about ix lxM;inches, placing it on edge between'the heads of the Olsen machine, andbending it, the beam being kept in place.

It was found that after the square had started to bend, its bendingstrength was fairly constant until it was deflected so that the edgeswere approximately one inch apart. It was also observed that a high loadis required to start bending which decreases sharply to a value somewhatabove that of the value applicable during the major part of the bendingoperation. In each instance, three parts of acid were added to thecontrol compound.

above outlined for mercaptobenzothiazole. Re sults were as follows:

It is evident that the control stock is a fast low temperature curingcompound and has a decided tendency to scorch. The addition of thesematerials results in a marked decrease in the scorching tendency of thestock, changing it from a stock impracticable to handle in the factoryto one of similar physical properties which can be handled with littleor no trouble. Further the addition of as much as one part by weight ofthese materials tends todecrease the tensile strength somewhat. This maybe largely overcome, however, by using smaller amounts.

Other examples demonstrating the scorch retarding properties of thesealiphatic carboxylic acids are found in the following tests conductedwith a compound containing butyraldehydeaniline as the accelerator.Inasmuch as the latter is a high temperature accelerator, these trialswere made with the adaptation of the scorch test formula to obtainoptimum cures at 310 degrees F. The compound used is as follows:

Parts by weight Rubber (pale crepe) 50.00 Rubber (smoked sheet) 50.00-Zinc oxide 10.00 Sulfur 2.50 Accelerator 1.25

To this basic formula, varying amounts of the retarding materials wereadded and after uniform mixing the stocks were cured at 310 degrees F.and subjected to physical tests. Index numbers a Show also were obtainedon the unvulcanized stock by Material Bending rometet the methodoutlined above. Results were as folstrength hardness lows:

16-9 lbs"- as ludexnum 24H 1b 4 Amounts Ultimate Maximum 28-12 lbZ- 85Material added tensile elong. bet 6 99-50 2520lbs 95 2616lbs 9s l8-l6lbs- 93 239 700 56 i2; 388 84 122 It is to be understood that thesecompounds are Rubber (pale crepe) 50.0 Rubber (smoked sheet) 50.0 Zincoxide 10.0 Pigment 10.0 Sulfur 3.0 Zinc dimethyl dithiocarbamate 0.4

To portions of this compoundwere added one part each of representativealiphatic carboxylic acids. Physical properties at the state of optimumcure and scorch tests were obtained as From the foregoing examples,illustrating the improved process of the invention, it will be seen thatthe addition of a small portion of an aliphatic carboxylic acid or metalsalt thereof to the rubber mix effectively prevents scorching attemperatures ordinarily met with in the milling of the rubber prior tovulcanization and that the compounds added to the stock do not interferewith the vulcanization of the rubber at higher temperatures. Theunvulcanized stock is stable and can be kept without the necessity ofcooling it to a low temperature and without the danger ofself-vulcanization during storage.

Another advantage of the invention is apparent in rubber cements whichare commonly used in various manufacturing processes, in particularthose cements which are self-curing or aircuring. One of the maindifliculties has been the gelling, or setting up, of the self-curingcement a short time after the preparation of the mixture,

making it necessary to use these cements within a very short time. Bythe use of the class of materials disclosed herein, this gelling isgreatly retarded.

Another advantageous use of the invention lies in the decreased amountsof softener required in rubber stocks. Heretofore, it has been necessaryin processing to use various percentages of certain rubber softeners,such as pine tar and pine oil, which supposedly by their softeningaction renders the stocks less scorchy. These softeners, however,detract from the quality of the rubber compound, especially in abrasiveresistance, and the more the softener, the less the quality as a generalrule. By the use of these scorch retarders, smaller amounts of softenersmay be used, the stocks being capable of processing at somewhat highertemperatures without the usual dangers of scorching.

Still another advantage, as above pointed out, lies in their use inhighly compounded, heavilyloaded stocks, wherein they increasematerially the hardness and resistance to bending.

From the foregoing it will be seen that the invention is of generalapplication in the vulcanization of rubber compounds, whether or notcontaining the usual compounding ingredients such as reinforcingpigments, activators, softeners, antioxidants, etc., all or any of whichmay be employed advantageously. It is of course desirable, although notstrictly necessary, to determine by test the preferred amounts in eachcase. It will be understood that numerous changes may be made in theinvention without departing from the spirit thereof. It is intended thatthe patent shall cover, by suitable expression in the appended claims,whatever features of patentable novelty reside in the invention.

. What I claim is:

1. The improvement in the prevention or retarding of the scorching ofrubber compounds containing an ultra or semi-ultra accelerator whichcomprises incorporating in the rubber compound a small amount of an acidhaving the formula (CH)2(COOH)2.

2. The improvement in the prevention or retarding of the scorching ofrubber compounds containing a thiazole accelerator which comprisesincorporating in the rubber compound a small amount of an acid havingthe formula (CH) 2(COOH) 2.

3. An unvulcanized rubber compound containing an ultra or semi-ultraaccelerator, said compound being stabilized against scorching by theaddition of a small amount of an acid having the general formula (CH)2(COOI-I) 2.

4. An unvulcanized rubber compound containing a thiazole accelerator,said compound being stabilized against scorching by the addition of asmall amount of an acid having the general formula (CH)2(COOH)2.

5. A method of enhancing the hardness and bending strength of highlyloaded rubber stocks containing an ultra or semi-ultra accelerator whichcomprises incorporating in the rubber compound a small amount of analiphatic poly carboxylic acid having the formula (CI-D2- (C0OH)2.

6. A method of enhancing the hardness and bending strength of highlyloaded rubber stocks containing a thiazole accelerator which comprisesincorporating in the rubber compound a small amount of an aliphatic polycarboxylic acid having the formula (CH)2(COOH)2.

7. The improvement in the prevention or retarding of the scorching ofrubber compounds containing a thiazyl sulphide accelerator, su1- phurand zinc oxide which comprises incorporating in the rubber batch at thetime of its mixing a small amount of fumaric acid.

8. The process of enhancing the hardness and bending strength of highlyloaded rubber stocks containing sulphur, a metallic oxide and an activeaccelerator which comprises incorporating in the rubber compound a smallamount of fumaric acid and vulcanizing.

9. A vulcanizable rubber composition retarded as to vulcanization atprevulcanization temperatures and vulcanizable at elevated temperatures,including a semi-ultra or ultra accelerator and fumaric acid.

HERMAN R. THIES.

